Optical pyrometry is well known technology in which radiation from an object, such as a jet engine fan blade, the temperature of which is to be measured, is projected by a lens and an optical transmission means to sensors sensitive to a selected wave length or wave lengths. The intensity or relative intensities of the radiation in a selected portion or portions of the spectrum is a measure of the temperature of the radiation emitting object. Because the pyrometer lens is often exposed to extremely harsh environments, the lens is subject to contamination by materials such as soot particles engine oil, etc. Such contaminants interfere with proper operation of the pyrometer because the presence of contaminating materials reduces the intensity of the radiation passing through the lens and, more importantly, reduces the intensities by different amounts in different wave lengths. In a two-color pyrometer where the ratio of the emitted radiation in different wave lengths is measured serious measuring errors can result.
In an application Ser. No. 943,001 entitled "Vortex Purge Air Cleaning Optical Pyrometer Lens," filed Dec. 18, 1986 in the name of Donald William Craft, the inventor of the instant invention, which application is assigned to the General Electric Company, the assignee of the present invention, a lens cleaning arrangement is described in which the cleaning or purge air is caused to flow spirally inward over the surface of the lens.
The purge air thus forms a vortex which has been found to be remarkably effective in cleansing the surface of the lens. However, the swirl or vortex cleaning arrangement described in the above identified application has certain limitations. The inward flowing swirling purge air becomes static and thus forms a stagnation zone at some radius from the axis of symmetry and air flow over the surface is no longer spiral or vortex flow but becomes rigid body fluid flow which moves outward along the axis of symmetry. Within the stagnation zone contaminating particles are deposited on the lens. Attenuation of radiation transmitted through this varies in different portions of the radiation spectrum, and this differential attenuation can result in erroneous readings.